1. Field of the Invention
The present invention relates to a driving circuit and a constant current driving apparatus using the same.
2. Description of the Related Art
Conventionally, a driving circuit which has a current mirror circuit has been used in order to drive a load at a constant current. FIG. 1 shows an example of such a conventional driving circuit. This driving circuit is composed of a current mirror circuit and a reference current setting resistor R.
The current mirror circuit is composed of a plurality of PNP transistors Tr0 to Trn. In this current mirror circuit, it is assumed that a power supply terminal and the respective PNP transistors Tr0 to Trn are physically arranged at positions shown in FIG. 1. In short, the PNP transistor Tr0 is physically arranged at the closest position to the power supply terminal, and the PNP transistor Trn is physically arranged at the farthest position from the power supply terminal.
Bases of the plurality of PNP transistors Tr0 to Trn are connected to each other. Emitters are commonly connected through a common power supply line to the power supply terminal, and collectors are connected to output terminals O1 to On, respectively. The base of the PNP transistor Tr0 arranged at a first stage of this current mirror circuit is connected to the collector of the PNP transistor Tr0 so that a so-called diode coupling is established.
In this current mirror circuit, a current of which value is substantially equal to that flowing through the collector of the PNP transistor Tr0, is flowed through each of the collectors of the PNP transistors Tr1 to Trn, and outputted as output currents from the output terminals O1 to On. Accordingly, a load that is set at an independent potential is current-driven. The collector of the PNP transistor Tr0 arranged at the first stage is connected through the reference current setting resistor R to a ground.
This reference current setting resistor R enables a reference current Iref flowing through the collector of the PNP transistor Tr0 to be adjusted. Thus, the suitable selection of the value of the reference current setting resistor R enables a current having a desired value to flow through the output terminals O1 to On. Hence, it is possible to drive a load requiring a constant current drive. This conventional driving circuit is generally configured, for example, as one semiconductor integrated circuit (IC).
Recently, as the load requiring the constant current drive, there are an LED display panel composed by arraying a plurality of light emitting diodes (hereafter, referred to as “LED”), an organic electroluminescence (hereafter, referred to as “EL”) display panel composed by arraying a plurality of organic EL elements using the electroluminescence phenomenon of organic compound, and the like.
In those display panels, the large number of LEDs and the large number of organic EL devices are used as light emission devices. Thus, a constant current driving apparatus can not be constituted only by one driving circuit (IC). Generally, the large number of light emission devices constituting the display panel are divided into a plurality of blocks, and the plurality of blocks are driven by a plurality of driving circuits, respectively.
In this case, when there is an irregular component in an output current outputted from the output terminal of each driving circuit, this irregularity causes a variation to be induced in a light emission amount of the light emission device. As a result, a display irregularity is generated in the display panel. Therefore, in order that the output current outputted from the output terminal of each driving circuit is made constant, the reference current is made constant by adjusting the resistive value of the reference current setting resistor R installed in each driving circuit.
As mentioned above, the conventional driving circuit employs the method of independently setting an input current of the current mirror circuit by adjusting the resistive value. Thus, when the plurality of driving circuits are used to drive the display panel at the constant current, it is difficult to reduce the variation in the reference currents of driving circuits.
In order to solve such problems, Japanese Laid Open Patent Application (JP-A 2000-293245) discloses “Constant Current Driving Apparatus And Constant Current Drive Semiconductor Integrated Circuit”. In this constant current driving apparatus, a plurality of constant current driver ICs are used in order to drive an organic EL element of an EL display panel at a constant current.
A constant current driver circuit and a control circuit are contained in each of the constant current driver ICs. A reference current generating circuit is built in each of the constant current driver ICs. A reference output current generated on the basis of a reference resistor is outputted from a reference terminal. The reference output current outputted from the reference current generating circuit is inputted to a reference current input terminal of each constant current driver IC. The drive currents having the same current value are respectively outputted from the output terminals. The drive current is controlled so as to be turned on and off by each control circuit.
Due to this configuration, by using the plurality of the constant current driver ICs, a large number of loads can be driven at the small variation in the output current among the constant current driver ICs.
However, in the case of the driving circuit constituted by the current mirror circuit having the large number of output terminals, the impedance of the common power supply line of the current mirror circuit causes a current ratio to be deviated on the basis of the position of the output terminal. FIG. 2 shows the relation between the deviation in the current ratio and the wiring resistance value of the current mirror circuit.
As can be understood from FIG. 2, as the wiring resistance value of the current mirror circuit is increased, namely, as the position of a cell (transistor) is located farther from the power supply terminal, the deviation in the current ratio is made larger. As a result, a difference occurs between a brightness of a light emission device driven by a transistor located close to the power supply terminal and a brightness of a light emission device driven by a transistor located far from the power supply terminal.
Thus, as shown in FIG. 3A, when the four driving circuits drive the light emission devices of the display panel, the output current of each of the driving circuits is decreased as they are located farther from the power supply terminal, as indicated by solid lines of FIG. 3B. This results in the large difference between an output current from an output terminal on an end side of one driving circuit and an output current from an output terminal on a beginning side of a driving circuit adjacent thereto, as indicated by A of FIG. 3B. This results in the evident brightness difference in the boundary between the light emission device driven by the one driving circuit and the light emission device driven by the driving circuit adjacent thereto. Hence, the picture quality is extremely dropped.
Dashed lines in FIG. 3B indicate the variation in the output current, which variation is generated based on the variation in the electrical characteristic caused by the product process of the driving circuit. Although, this problem can be logically solved by suitably selecting the reference current by properly selecting the value of the reference current setting resistor R, actually, it is difficult to suppress the variation in the reference current to a small value, as mentioned above.
In order to solve the above-mentioned problems, namely, in order to reduce a slope of a property line of FIG. 2, it is necessary to reduce the impedance of the common power supply line. For this purpose, it is necessary to make the width of the common power supply line wider or install a plurality of power supply terminals. Also, in order to reduce the variation in the reference current of the current mirror circuit, it is necessary to carry out a trimming and the like or manage the semiconductor integrated circuit constituting the driving circuit at a wafer unit. They result in the factor of the cost increase when the driving circuit is constituted by the semiconductor integrated circuit.